Image intensifier devices multiply the amount of incident light they receive and provide an increase in light output, which can be supplied either to a camera or directly to the eyes of a viewer. Image intensifiers are constructed for a variety of applications and hence vary in both shape and size, with proximity focused image intensifiers comprising a particular type of image intensifier having the smallest size and weight of all categories of image intensifiers. These devices are particularly useful for providing images from dark regions and have both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are employed in security cameras and in medical instruments to help alleviate conditions such as retinitis pigmentosis (night blindness). Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780 entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips issued on Jan. 28, 1992 and assigned to ITT Corporation the assignee herein.
Image intensifiers include active elements, support elements and supply elements. The active elements include the photo-cathode (commonly called simply "cathode"), micro-channel plate (MCP), phosphor screen (screen), and getter. The cathode detects a light image and changes the light image into an electron image. The MCP amplifies the electron image and the screen changes the electron image back to an light image. The getter absorbs gas which is generated during operation of the tube.
The support elements comprise the mechanical elements which physically support the active elements of the tube. In a standard proximity focused tube these support elements are the vacuum envelope (known as the body), input faceplate (sometimes also called "cathode"), and the output faceplate or fiber-optic (also called "screen").
The supply elements in the tube include the chrome contact that is deposited on the faceplate to the cathode, the screen aluminum contact which is deposited on the fiber-optic or output faceplate, and the metalizing on the MCP glass. In addition the metal parts in the body assembly also provide electrical contact.
Finally there are packing elements which perform other functions. The fiber-optics direct the image generated by the screen to a convenient position so that the system optics can properly direct the image to the ocular plane.
As is known, three major components of modern image intensifier tubes are the photocathode, phosphor screen (anode), and MCP disposed between the photocathode and anode. These three components are positioned within the evacuated housing or vacuum envelope, thereby permitting electrons to flow from the photocathode through the MCP and to the anode. In order for the image intensifier tube to operate, the photocathode and anode are normally coupled to an electric source whereby the anode is maintained at a higher positive potential than the photocathode. Similarly, the MCP is biased and operates to increase the density of the electron emission set forth by the photocathode. Furthermore, since the photocathode, MCP and anode are all held at different electrical potentials, all three components are electrically isolated from one another when retained within the vacuum housing.
Two major disadvantages are associated with the prior art image intensifiers. The first disadvantage concerns the interface with the image intensifier system, notably the objective lens and the eyepiece. The second disadvantage concerns the length and complexity of the tube and its housing, which causes problems particularly for user's of night vision goggles. The major interface problem with the present intensifier tubes is the input faceplate thickness which is typically 0.210" thick. The input faceplate is part of the optical elements included in the image intensifier tube's objective lens. As an optical element it introduces defects in the image called aberrations. These aberrations reduce the resolution and contrast of the system. The aberrations from the faceplate can be corrected by introducing more lens elements, increasing the index of refraction of the present elements, or using non-spherical curves in the elements. However, each of these approaches increases the weight and cost of the objective lens and thereby the system. In addition, if the optical path of the objective lens is folded by a mirror or prism, the thick faceplate can not be brought into proper focus.
The second interface problem is stray light reflecting off of the slope on the faceplate creating ghost images and lower contrast. In the prior art, this slope is required in the tube so that the photocathode, which is mounted on the resulting surface, is in focus for the MCP. Finally, in the case of night vision goggles, the total length of the tube pushes the objective lens in away from the head causing the user to perceive that the goggle system is heavier than its actual weight. Thus, shortening the length of the tube is highly desired.
The fundamental reasons that the tube is long in prior art devices are that while the cathode, MCP and screen must be in close proximity to each other to give a high resolution image, the high voltages required to operate the device must have a certain amount of physical path distance so that leakage or breakdown do not occur. Furthermore, ceramic parts and shields are added for supplying the getter for long product life. The ceramic spacers and hold down mechanisms for the MCP are also pivotal in extending tube length. They are required to hold the MCP in its position and provide the electrical energy to the plate without breakdown. As a result the sloped section of the faceplate is required to place the cathode in proximity to the MCP so that a chrome contact must be used. This requires additional metallization deposition steps for fabricating the image intensifier. These parts add approximately 0.09" in tube length. The need for a getter to absorb the gas adds approximately 0.06" in length to the tube. These and other miscellaneous requirements yield a tube length of approximately 0.7" long.
In view of the prior art, there exists a need for an improved image intensifier tube having a thin flat faceplate to reduce optical aberrations caused by sloped cathodes as well as reducing tube length. The photocathode should directly contact the support ring to provide an electrical bias so as to eliminate the chrome metal deposition process for sloped photocathodes. Furthermore, an improved housing is desired which can further reduce tube length and retain and support the MCP while electrically isolating the photocathode, MCP and anode from one another.